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Abstract:

Exceptionally well-preserved arthropod cuticle from the Mount Cap Formation (Northwest Territories) is described on the basis of several thousand specimens. Various elements of crustacean-type mouthparts are identified, including the molar (grinding) surfaces of mandibles, some 53 categories of cuticular projection, and complex setal armatures indicative of precise and versatile particle-handling abilities. The reconstructed feeding apparatus of the Mount Cap taxon is fundamentally more sophisticated than those of its contemporaries, and thus substantially expands the known ecological repertoire of Cambrian arthropods, while specific characters of the mandibular ornamentation indicate a phylogenetic position within the crown group of Pancrustacea, and provide the strongest evidence for this clade earlier than the late Cambrian. Scaling from the size of the mandibles predicts an overall body length of several centimetres. This suggests that the absence of comparably fine-scale anatomical detail from non-microscopic taxa preserved elsewhere in the Cambrian fossil record has strongly biased the reconstruction of early arthropod evolution. A preliminary systematic account is provided of a diverse organic-walled microfossil assemblage from the Forteau Formation of western Newfoundland. Important new fossils include geometric clusters of interconnected “acritarchs” that may represent an independent origination of green algal multicellularity, and organic components of hexactinellid-type spicules which contribute crucial data to the emerging pattern of extinct combinations of spicule characters among early sponges. The recovery of diverse and well-preserved organic microfossils from the Forteau shales, reveals that small-scale organic preservation is more widespread than its macroscopic expression in the form of Burgess Shale-type assemblages. “Small organic fossils” may thus emerge as a comparatively continuous source of powerful palaeobiological data across this key period in Earth history.